<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
</head>

<style>
    .fade-enter-active, .fade-leave-active {
        transition: opacity .5s;
    }
    .fade-enter, .fade-leave-to /* .fade-leave-active below version 2.1.8 */ {
        opacity: 0;
    }
</style>

<body>
<div id="demo">
    <button v-on:click="show = !show">
        Toggle
    </button>
    <transition name="fade">
        <p v-if="show">hello</p>
    </transition>
</div>

<script type="text/javascript">
    new Vue({
        el: '#demo',
        data: {
            show: true
        }
    })
</script>
</body>
</html>